The present invention relates to a flat-panel display device, and more particularly, to a flat-panel display device and manufacturing method therefor, in which display characteristics are improved by maintaining a uniform cell gap between upper and lower substrates.
Cathode ray tubes (CRT), and flat-panel display devices such as liquid crystal displays (LCD), plasma displays (PDP), electro-luminescent displays (ELD), field emission devices (FED), and light emitting diodes (LED) are currently used as picture display devices.
The CRT has excellent picture quality and brightness compared to other devices. However, the volume and weight of the CRT are great, so that it is difficult to use for a large screen display.
On the contrary, the flat-panel display device is now widely used due to its light weight and small volume. Also, research into the flat-panel display device as a display for the next generation has been actively performed.
Particularly, the LCD is a display device using peculiar properties of liquid crystals. The liquid crystal material is advantageous in handling and has a characteristic in which the alignment of the liquid crystals is changed according to the application of an external electrical field. Thus, liquid crystal materials are widely used in ferroelectric liquid crystal display devices (FLCD), twisted nematic LCDs (TN-LCD), thin film transistor LCDs (TFT-LCD), and plastic LCDs.
FIG. 1 is a sectional view showing the structure of a conventional LCD. First, indium tin oxide (ITO) electrodes 12 and 12' and, alignment layers 13 and 13' are sequentially stacked on the transparent substrates 11 and 11', respectively, to form first and second substrates 10 and 10'. Then, a spacer 14 is scattered between the alignment layers 13 and 13' of the first and second substrates 10 and 10'. Then, the first and second substrates 10 and 10' are sealed using sealant 16, resulting in a cell gap therebetween. Lastly, liquid crystal material is inserted into the cell gap to form a liquid crystal layer 15.
In the LCD having the above structure the alignment of the liquid crystals is changed by the application of an external voltage, and thus the light incident on the liquid crystal layer is blocked or transmitted. That is, when an electrical field is formed in the liquid crystal layer by applying a voltage to the transparent electrode, the liquid crystals are aligned in a predetermined direction, and thus the light incident on the liquid crystal layer is blocked or transmitted according to the alignment pattern of the liquid crystals. Such driving characteristics of the liquid crystals are largely influenced by the interval of the liquid crystal cell gap of the LCD. That is, since the physico-chemical reaction of the liquid crystals is determined according to the intensity of the applied voltage and the distance between the two electrodes, the physico-chemical reaction of the liquid crystals with respect to the voltage are changed and the transmittance ratio is not uniform if the thickness of the liquid crystal layer is not uniform. Thus, it is very important to maintain the interval of the cell gap in the LCD at a predetermined distance for obtaining a liquid crystal layer having a uniform thickness in manufacturing the LCD.
However, in a conventional LCD, circular or cylindrical spacers having larger diameters than the interval of the intended cell gap are scattered on the alignment layer of one of two substrates on which a transparent substrate, a transparent electrode and an alignment layer are sequentially deposited. Then, the other substrate is put on the substrate such that the alignment layers of two substrates face each other, and then a sealant as an adhesive material is applied at the edges of the two substrates. Therefore, the two substrates are sealed under pressure while applying heat or irradiation with ultraviolet rays, forming a cell gap.
However, if the interval of the cell gap is controlled by the above manner, various problems occur. First, since spacers are scattered irregularly, they may be partially agglomerated, resulting in a deviation in the interval of the cell gap. Further, the diameters of the spacers are non-uniform, so that it is difficult to control the interval of the cell gap. Also, since the spacers are not fixed within the cell gap, they flow during the injection of the liquid crystal material. Accordingly, the alignment layer may be damaged. Further, the electrodes may be damaged by the spacers when two substrates are sealed under pressure. Thus, the resultant LCD does not have good light blocking and transmittance characteristics.
To solve the above problems, a spacer formation method using a photolithography technique has been suggested. According to this method, a photosensitive material is deposited on the substrate to form a photosensitive layer, and then the photosensitive layer is exposed to light and developed, resulting in spacers having the intended pattern. However, this method may cause damage to the alignment layer.
On the other hand, a color LCD includes a first substrate including red, green and blue color filters as three principal colors of light, a second substrate including an active circuit portion with a thin film transistor, and a liquid crystal layer between two substrates.
FIGS. 2 and 3 show the structure of the first substrate including a color filter layer in a color LCD. The process of forming the first substrate will now be described. First, a light shielding black matrix 22a (see FIG. 2) is formed on a transparent substrate 21. Next, a photosensitive acryl resin including a dye with a spectroscopic property of red is deposited on the entire surface of the substrate 21, and then a red filter 23 is formed through baking, light-exposure and developing processes. A green filter 24 and a blue filter 25 are formed in the same manner as that of the red filter 23, thereby resulting in a color filter 20. The color filter may be in the form of strips, dots, or a mosaic.
FIG. 3 shows a substrate in which a black matrix 22b is formed after the step of forming a color filter layer 20.
Then, a protective film 26 may be formed of a transparent resin having strong surface hardness and excellent light transmittance in order to protect the black matrix 22a or 22b and the color filter layer 20 from external impact.
Next, a transparent electrode layer 27 for driving the liquid crystal is formed and then an alignment layer 28 is formed on the transparent electrode layer 27, completing the first substrate.
In the color filter of the LCD manufactured according to the above process, three wavelengths of light emitted from a fluorescent lamp pass a filter layer for selectively transmitting only a predetermined wavelength of light via a liquid crystal panel which is opened or closed by an electrical signal, so that a predetermined color (image) is achieved.
On the other hand, the plasma display panel displays an image using a gas discharging phenomenon, which is excellent in display capacity, luminance, and contrast. Also, there is little afterimage and the viewing angle is wide. Thus, the plasma display panel attracts attention as a next generation display device.
Generally, the plasma display panel is manufactured by the following steps: first, two transparent substrates made of a transparent material such as glass are prepared. On one of the transparent substrates, a transparent electrode in a stripe shape with a predetermined interval, a bus electrode in a stripe shape whose width is narrower than that of the transparent electrode, and a dielectric layer covering both the transparent electrode and the bus electrode are formed sequentially to complete a front substrate. On the other transparent substrate, an address electrode in a stripe shape which is orthogonal to that of the transparent electrode and a dielectric layer covering the address electrode are sequentially formed to complete a rear substrate. Also, barrier walls for maintaining a gap between two substrates to a predetermined level is formed between two dielectric layers of the front and rear substrates.
In a conventional plasma display panel, the barrier walls are formed by repeating a screen printing process several times until the height of the barrier wall reaches a predetermined level. However, the height of the barrier wall obtained by this method is not even, so that the cell gap between the upper and lower substrates is not uniform. Thus, an electrical and optical blocking effect between adjacent cells is not achieved.
Besides the above printing method, a sand blasting method is used. The sand blasting method is however complicated, and the yield therefrom is very low.
On the other hand, a laser transcription method was developed for the printing, typesetting, and photographic fields thirty or more years ago. According to the laser transcription method, a transcription substance, e.g., dye or pigment, included in a layer formed on a base film as a support is transcribed on a receiving film (glass or polymer film) according to an intended film pattern (U.S. Pat. Nos. 3,787,210 and 5,326,619).
Referring to U.S. Pat. No. 3,787,210, a mixture of the transcription material such as dye and pigment and nitrocellulose decomposed by light is deposited on a base film. As a result, the pigment or dye can be transcribed on a substrate by the explosive force of gas generated from the nitrocellulose through the thermal decomposition.
However, since such transcription process consumes much energy, a more effective and stable transcription process is required. As a result, a donor film has been developed. Here, the structure of the donor film is dependent on the thickness and physical properties of the transcription substance and its energy source. The donor film has a structure in which a light absorption layer for providing transcription energy through a thermal decomposition reaction by absorbing light, and a transcription layer including a transcription substance are stacked on a film functioning as a support. Here, the light absorption layer having a thickness of about 1,000 .ANG. absorbs light and transcribes the transcription substance using the explosive force of nitrogen or hydrogen gas generated during the thermal decomposition reaction.
The above laser transcription method will now be described in detail with reference to FIG. 4 schematically showing a transcription apparatus used in a general laser transcription method.
In FIG. 4, a high power laser beam is emitted from an energy source 41. As the energy source emitting power at a rate of 0.1.about.4 W, a high power solid laser such as Nd/YAG, gas laser such as CO.sub.2 and CO, or a diode-coupled Nd/YAG can be used. The emitted laser beam is divided into a plurality of beams having the same intensity via a beam splitter 42. If the intensity of the beam is controlled by dividing the beam into a plurality of beams, a substance can be transcribed in a desired shape (U.S. Pat. No. 4,796,038).
The intensity of the plurality of divided laser beams is controlled by a modulator 43 according to an intended shape, and then the laser beam irradiates a donor film 46 on which the transcription substance is deposited, via a condensing optical system 44. Here, only the substance deposited on a light-receiving portion of the donor film is transcribed onto a substrate 47. The movement of a stage 48 is controlled together with a raster 49 for controlling the intensity of the bundle of the beam according to the shape of the desired pattern.
The inventors of the present invention have conducted research into a method for forming a flat-panel display device in which a cell gap between upper and lower substrates is uniformly maintained using the above-described transcription method, thereby improving the display characteristics of the flat-panel display device.